Shuttle Exhaust Reveals More About Atmospheric Winds
August 29, 2012

Final Space Shuttle Launch Caused Bright Arctic Clouds To Form

April Flowers for - Your Universe Online

The last space shuttle flight took place on July 8, 2011, sending Atlantis into space for its thirty-third, and final mission. This flight marked the end of a 20-year space shuttle program. As Atlantis reached a height of about 70 miles over the east coast of the U.S., it released 350 tons of water vapor exhaust.

The vapor plume spread and floated on air currents high in the Earth's atmosphere, crossing through the observation paths of seven separate sets of instruments. An international consortium of scientists, led by Naval Research Laboratory's (NRL) Dr. Michael Stevens, tracked the plume to learn more about airflow in the Mesosphere and Lower Thermosphere (MLT) — a region that is usually difficult to study. Understanding the rapid transport of such high altitude exhaust plumes can provide insight into the effects of winds at the bottom edge of the space weather regime. This knowledge could be critical in improving models of communication signal propagation and over-the-horizon radar.

The study, published online in the Journal of Geophysical Research on August 27, found that the water vapor spread much faster than expected. Within 21 hours, much of it collected near the Arctic where it formed unusually bright high altitude clouds known as polar mesospheric clouds (PMCs). The results of this study will help improve global circulation models of air movement in the upper atmosphere and expand ongoing studies of PMCs.

"Polar mesospheric clouds are the highest clouds on Earth," says Stevens. "They shine brightly when the sun is just below the horizon and typically occur over polar regions in the summer. There is some evidence that they are increasing in number and people want to know if this is indicative of climate change or something else that we don't understand."

PMCs are also known as noctilucent clouds because they glow at night. They can serve as indicators of temperature changes, as well as how currents and waves move high in Earth's atmosphere. A visible cloud of water vapor such as the shuttle drops offers a serendipitous way to observe such motions.

"The plume from the shuttle becomes a ready-made experiment to observe the movement in the atmosphere," says Charles Jackman, project scientist for NASA's Aeronomy Ice in the Mesosphere (AIM). "What this team found is interesting since the plume moved so quickly to the pole, indicating that the winds appear much stronger at those latitudes than was thought."

Of the seven instrument sets to observe this plume as it moved through, the first two were on a NASA spacecraft called TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics). Next in line was the Sub-Millimeter Radiometer on the Swedish Odin satellite. As the plume reached higher latitudes, it was picked up by the ground-based Microwave Spectrometer at the Institute of Atmospheric Physics in Kühlungsborn, Germany, as well as an identical ground-based water vapor instrument called cWASPAM1 at the Arctic Lidar Observatory for Middle Atmospheric Research (ALOMAR) in Andenes, Norway. The plume collated into its final shape over the arctic, as a new, extremely bright PMC on July 9, 2011 and there, it could be observed from above by the AIM satellite flying overhead, and from below by another instrument at ALOMAR called the RMR lidar.

This series of observations showed the plume spreading horizontally over a distance of some 2000 to 2500 miles. Some parts of the plume drifted near the North Pole and formed ice particles which settled into layers of PMCs down at about 55 miles above Earth's surface. The quickness with which the plume reached the arctic was a surprise.

"The speed of the movement in the upper atmosphere gives us new information for our models," says Stevens. "As you get higher up in the atmosphere, we just don't have as many measurements of wind speeds or temperatures. The take-away message here is that we need to improve the models of that region."

Current theories suggest that the plumes are rapidly transported because of narrow, high-speed wind shears. These wind shears are also linked to the occurrence of so-called Sporadic E events, thus establishing a possible link between plume transport and the lower ionosphere.

A Sporadic E event is an unusual form of radio propagation using characteristics of Earth's ionosphere in which radio signals bounce off smaller "clouds" of unusually ionized atmospheric gas in the lower E region, allowing for the occasional long-distance communication at VHF frequencies not usually well suited to such.

Observations of PMCs may be connected to global climate, so it becomes important to subtract out sporadic effects such as shuttle exhaust from other consistent, long-term effects.

"One of AIM's big goals is to find out how much of the cloud's behavior is naturally induced versus man-made," says Jackman. "This last shuttle launch will help researchers separate the shuttle exhaust from the rest of the observations."

The AIM observations show a clear difference between the shuttle-made PMC and typical ones. Normally smaller particles exist at the top, with larger ones at the bottom. The shuttle plume PMC showed a reversed configuration with larger particles at the top and smaller ones at the bottom. This offers a way to separate out such clouds in the historical record. Shuttle made clouds are also brighter than over 99% of all other PMCs.

Image 2 (below): The Cloud Imaging and Particle Size experiment on NASA's Aeronomy of Ice in the Mesosphere satellite observes PMCs about ten times brighter than usual over Scandinavia the day after launch of STS-135. Water vapor exhaust from the shuttle and other rockets may have led to significant PMC production of the past three decades, complicating the use of PMC occurrence as an indicator of upper atmospheric climate change. Credit: U.S. Naval Research Laboratory